This invention relates to a process for the reaction of metal chlorides with sulfur dioxide and/or sulfur trioxide gas in a truly fluidized state, with the production of a solid having a composition composed of the metal and one or more constituents of the sulfur dioxide or trioxide and a gaseous acid. More particularly, the invention relates to the production of potassium sulfate fertilizer and hydrochloric acid using sulfur dioxide or sulfur trioxide gas and potassium chloride in an energy efficient and non aqueous medium using counter current flow and true fluidized bed technology. The unique aspects of this process is that it permits the reaction to take place at a very rapid rate (minutes versus days) at moderately elevated temperatures while retaining essentially the same crystal size and screen analysis as the original potassium chloride. The counter current aspects of the invention permit the production of high purity potassium sulfate and a hydrochloric acid gas essentially free of sulfur dioxide.
Potassium is one of three essential elements (N.P.K.) in the life cycle of all plants. Fertilizers therefore generally contain all three in one form or another. Potassium, however is generally present as a chloride since it is the most readily available, least expensive potassium compound. For many crops (e.g., citrus, tobacco) a fertilizer containing small amounts of chlorides is toxic. Thus, there is created a sizable demand for manufactured potassium sulfate as a non chloride source of potassium. However, it must be produced at a relatively low cost to compete with existing processes such as that produced from natural deposits and brines. Potassium sulfate is not known to occur in nature except as a double salt xe2x80x9cK2SO42MgSO4xe2x80x9d, (langbeinite), which is mined and sold as such. While the magnesium is a desirable micronutrient, the proportion present in the langbeinite 15.25% reduces its desirability as a substitute for potassium chloride.
In addition langbeinite generally occurs with interstitial sodium chloride xe2x80x9cNaClxe2x80x9d and requires careful washing and size control to produce a product of ninety five percent langbeinite. Pure potassium sulfate can be produced from langbeinite by reaction with potassium chloride:
K2SO42MgSO4+4KClxe2x86x923K2SO4+2MgCl2
However, the process requires dissolution of the solid followed by evaporation and crystallization to recover potassium sulfate. Control of the process is difficult and the product produced is borderline in purity and particle size.
Hargreaves, et. al taught in U.S. Pat. No. 149,859, Apr. 24, 1874, that sodium and potassium chloride could be converted to the sulfate salts and hydrochloric acid by contacting the chloride salts with sulfuric acid made by a process similar to the old chamber process involving oxidation of sulfurous acid with air and water vapor by increasing the rate of conversion of sulfurous acid to sulfuric acid by the addition of niter or nitric acid. The conversion of the chlorides was accomplished in a rather crude vessel by forcing or inducting the sulfuric acid vapor through a fixed bed of the alkali salts until conversion was complete. Heat was added to the air, sulfurous acid and water before, during, and after mixing to elevate (superheat) the gases and acid vapors. Many processes utilizing this fundamental reaction have been proposed. A number of these use the term xe2x80x9cfluidized bedxe2x80x9d to describe spouting beds similar to those produced in xe2x80x9cWursterxe2x80x9d type apparatus, or to describe a xe2x80x9cdense phase gas agitated bed in a state of substantial fluidityxe2x80x9d or to describe xe2x80x9can allutriated zonexe2x80x9d, or to describe gas conveyed or entrained solids. While these may fall under the generic term xe2x80x9cfluidizedxe2x80x9d they are not true or conventional fluidized beds. xe2x80x9cIn a true fluidized bed particles are kept in a randomly moving, fluidized condition by a stream of pressurized gas. This is usually accomplished by placing the particles on a perforated support usually a metal plate or screen. A pressurized gas is forced through the perforations in the plate and causes the particles to fluidize. True fluidization is characterized by the particles moving in a random turbulent fashion similar to a gently boiling liquidxe2x80x9d. (See Darrah et. al. U.S. Pat. No. 5,399,186 Mar. 21 1995 column 4, line 44). In Nguyen U.S. Pat. No. 4,495,163 Jan. 22, 1985 column 3, line 34, the author found it necessary to define fluidized beds generically as follows to distinguish his process. xe2x80x9cThe term fluidized bed as used herein is intended to include conventional fluid beds, fast moving fluid transport systems wherein the pellets are carried in the gas stream separated and returned to a point of introduction, spouting beds, etcxe2x80x9d. It is important to emphasize this distinction since true or conventional fluidization causes the solids to flow and act like liquids. True fluidization as used in the instant invention is that which meets the definition outlined in the Darrah patent, which permits the use of counter current gas and solid flows, which results in the production of high purity potassium sulfate and hydrochloric acid, retention of crystal integrity, rapid conversion rates, simplicity of the apparatus, very efficient energy conservation and the elimination of environmentally controlled emissions, and excludes sporting beds, dense phase gas agitated beds, allutriated zones or fast moving transport systems since using such means other than that described as conventional or true fluidization will not accomplish the results described.
The Potash Company of America used the Hargreaves process to produce potassium sulfate from potassium chloride utilizing sulfur dioxide in a small plant at Dumas, Tex., now shut down as commercially unprofitable. This process required the compaction of approximately 1 ounce of pulverized 100 mesh potassium chloride into oval shaped briquettes approximately 2 inches long by xc2xe inches thick, which are dried in a gas fired rotary dryer, screened, and stored prior to loading into a conversion chamber. The plant contained 8 conversion chambers which were brick lined pits 16 feet square and 14 feet deep to a floor of cast iron grates on which approximately 50 tons of briquettes were distributed. The chambers were covered by a 1 piece insulated steel cover. Seven of the chambers were maintained in operation while one was being unloaded and reloaded with new briquettes.
Heavy 24 inch piping connected the chambers in series so that the sulfur dioxides gas produced by burning sulfur in the presence of air and water vapor entered the bottom of each chamber and left at the top at a diagonally opposite corner. The chamber in operation the longest (the first chamber) received the strongest gas, while the newly charged chamber received the weakest sulfur dioxide gas. The first chamber in the series was unloaded and reloaded each day so that 7 chambers were in operation at any one time. The second chamber then became the first in the series, the new chamber the last with the intermediate chambers moving up in place in the series. The process had to be shut down each day to connect the newly charged chamber and remove the oldest chamber for unloading. Piping had to be rerouted to accommodate this charging and unloading process. Briquettes unloaded from the chamber were crushed and screened, providing a significant amount of fines. Considerable energy was also used to dry the compacted briquettes, which were formed from wetted potassium chloride. Hydrochloric acid, produced by the reaction, was sold as a commercial grade.
Aside from the inefficiency associated with daily shut down and start up of the process, it is also obvious from the description of this process that it was very labor intensive and inefficient in this respect. The integrity of the initial potassium chloride crystals was also lost because of the mechanical grinding prior to briquette compaction and following conversion which resulted in uncontrolled production of fines. In addition, incomplete penetration of the gas to the center of the briquette often left unreacted KCL which contaminated the finished product. Also because of the significant pressure drop between chambers, blowers were spaced throughout the series to maintain gas flow. These blowers were subjected to hot hydrochloric acid and sulfur dioxide gases resulting in high maintenance requirement both for interconnecting piping and blowers. The instant invention by means of simplicity and the use of truly fluidized bed techniques eliminates all of the foregoing disadvantages and inefficiencies and results in a product of superior granular quality and product purity.
Lippman et al, in U.S. Pat. No. 2,336,180 disclose a process for manufacturing alkali metal sulfates by dispersing or injecting a cloud of finely divided alkali metal chlorides into a moving stream of sulfur dioxide, oxygen (air) and water vapor at reaction temperatures of approximately 1400xc2x0 F. to 1550xc2x0 F. There are many inherent disadvantages to this process, among them being the use of xe2x80x9cchloride dustxe2x80x9d as feed material so as to accelerate the reaction rate and hopefully result in complete reaction in the time permitted in the moving gas stream. High temperatures in the range of 1550xc2x0 F. are required for complete reaction for particles in the range of 63% through 100 mesh and 35% through 200 mesh which is very fine particulate distribution. Obviously, very fine feed material will result in very fine or powdered finished product, which is commercially undesirable. Temperatures as high as 2000xc2x0 F. or higher are required if the particles are more coarse. If more desirable lower reaction temperatures are required a catalyst (an impurity) of iron oxide (0.5%) is added. Since the alkali metal chlorides are injected into the moving gas stream the flow of gas and particles are concurrent except at the point of injection where some momentary counter current flow possibly exists due to gravitational pull on the particles, and which is almost immediately followed by capture of the particles in the moving gas stream. There is very little time therefore for complete reaction to occur and furthermore as the gas and particles proceed concurrently up the tower the sulfur dioxide concentration gradient between the gas and the particle diminishes due to adsorption of SO2 on the surface of the particles, desorption of SO2 from the gas stream, and the addition of hydrochloric acid gas to the gas stream. This process by its very nature thus increases the possibility of unreacted halide remaining in the center of the particle.
In addition, as stated in the patent xe2x80x9cthe cloud of products evolved from the top of the furnace comprises essentially sodium sulfate, hydrogen chloride, nitrogen, residual oxygen with small amounts of sulfur dioxide sulfur trioxide and unconverted salt.xe2x80x9d Expensive gas separation methods must be employed to recover SO2/SO3 free hydrochloric acid and the finished sulfate product will be contaminated with chlorides and possibly iron if it is used as a catalyst. The recovered muriatic acid will also be contaminated with dissolved SO2 (sulfurous acid) and SO3 (sulfuric acid). Since SO2 is not readily soluble an air pollution hazard is also possible from unabsorbed SO2.
Cannon in U.S. Pat. No. 2,706,144 teaches a method for making sulfate salts and hydrochloric acid by reacting sodium, potassium, calcium chlorides in a deep (20 ft) dense bed agitated by a gas stream containing sulfur dioxide, air, and water vapor passing upwardly through the bed. Conversion of the chloride to sulfate takes place on the particle surface and is ostensibly abraided off by the agitation of the particles. The sulfate product is a very fine dust which is entrained in the upward moving gas stream. Cannon describes two processes, a continuous and batch operation both producing this very fine sulfate, which because of this characteristic, has limited commercial demand. The batch operation depicts cyclone separators as being required to recover this fine dust whereas in the continuous operation cyclones are not indicated and separation of the fine dust occurs by gravity in a reduced diameter central tube.
Since the gas velocity in the large diameter outside disperse phase is conveying this dust into a much smaller central tube the gas velocity in this smaller diameter tube and in the tubing exiting the apparatus will be significantly increased, thus causing a significant amount of the sulfate to be entrained in the gas and entrapped in the salt preheater or as a contaminant in the hydrochloric acid. Sulfate entrapped in the preheater will be reintroduced into the dense phase and again contacted by the reacting gases and subject to reaction (4, column 1, ln 46) introducing a possible impurity in the finished product. The final sulfate product does not retain the size integrity of the original metal chloride, the fine dust produced by the process description is significantly more fine than the original 120 mesh feed which is itself quite fine. Another significant disadvantage for both processes is that the SO2 concentration gradient between gas and solid decreases upwardly through the column increasing the probability of SO2 contamination of the hydrochloric acid, or the environment if the SO2 is not absorbed in the acid. This is very likely to be a serious problem in the batchwise operation since the bed height decreases as the reaction proceeds and the SO2 concentration gradient between the solid and gases also decreases. Energy efficiency is decreased due to the necessity to preheat the salt feed particularly during start-up of both processes. A further disadvantage is the time required to produce sulfate product which the process description indicates as taking hours to complete the reaction.
Cannon also discloses in U.S. Pat. No. 2,706,145 a method of converting chlorides of a metal selected from the group consisting of sodium, potassium, and calcium to a metal sulfate selected from the group consisting of sodium, potassium, calcium and hydrochloric acid. This Cannon process requires the use of vaporized sulfuric acid heated to high temperatures in a deep (15 to 20 feet) dense agitated bed to produce by abrasion a fine dust of metal sulfate. This process is subject to the same deficiencies as those previously outlined in U.S. Pat. No. 2,706,144. Furthermore, this process is energy inefficient due to the large quantity of heat required to preheat the metal chlorides and vaporize the sulfuric acid. Serious corrosion problems also exist due to the corrosive nature of hot sulfuric acid vapors.
Cannon in U.S. Pat. No. 3,563,701 teaches a method of producing metal sulfates and volatile acid gases by reacting salt particles containing the significant element of the acid gas with a spray of sulfuric acid in a reaction zone maintained at high temperatures. As indicated in the xe2x80x9cBackground of the Inventionxe2x80x9d section of this patent column 1, line 49, the author emphasizes a basic weakness in his prior teachings, as outlined previously, namely the production of xe2x80x9cexcessively dustyxe2x80x9d fines. This current patent addresses this issue by employing a spouting suspended bed of salt particles transported in extremely hot gases (1600xc2x0 F. to 2000xc2x0 F.) and into which sulfuric acid liquid and vapor is sprayed in a concurrent fashion. Conversion to the sulfate occurs in the brief residence time in the reactor. Agglomeration of a percentage of the particles does occur because of the extreme temperatures employed in the presence of liquid sulfuric acid. However, since there is no means to reinject unreacted particles back into the reaction zone as in a Wurster type apparatus, unreacted salt particles and small sulfate particles gravitate into the product discharge line. Spargers are installed in three locations to elutriate these particles back into the reaction zone. Since the velocities in this zone are very high and turbulent it is suspected that particles suspended in the elutriating air are carried up into the area where the hot gas jet has expanded into the full diameter of the vessel and are then conveyed by the reaction gases out of the reactor and recovered by the cyclones together with unreacted fine chloride and sulfate carried up from the reaction zone. These chloride contaminated fines have little commercial value and must be recycled to the reactor. The process description is silent respecting the percentage which this recycle contributes to the feed however because of the brief reaction time a significant percentage of the feed is likely to be unreacted, partially reacted or only partially agglomerated. This could constitute a high recycle rate and a diminished efficiency.
It is also suspected that contamination of the fines and the product hydrochloric acid by sulfuric acid also exists. The process description calls for stoichiometric proportions of sulfuric acid and salt feeds into the reaction zone (column 5 line 30-34) whereas it also states that the reaction may be essentially carried out to completion with 93 to 98 percent stoichiometric yields. This means that 2 to 7% of the stoichiometric amount of sulfuric acid and potassium chloride do not end up as a constituent of the sulfate product, therefore they must end in the hydrochloric acid product or as a contaminant in the sulfate salt. This contamination would seriously reduce the commercial value of the finished products.
The apparatus is complex, containing small passages, subject to plugging unless stringent process controls are employed (preventing salt from softening/melting etc.) The apparatus is subject to extremely corrosive conditions (hot sulfuric acid vapors and liquids mixed with gases at 1600xc2x0 F. to 2000xc2x0 F.) Finally large amounts of heat are required to sustain the operation by preheating the feed salt, the entraining gases (1600xc2x0 F. to 2000xc2x0 F.) and vaporizing and preheating liquid acids thus substantially adding to cost of manufacture.
Cannon in a subsequent U.S. Pat. No. 3,717,440 simply claims the apparatus used in the previously described art.
Potassium sulfate is also recovered from saline lakes or seas (e.g., Great Salt Lake). Concentration of salts in some of these brines has been so affected by rainfall that production was suspended at Great Salt Lake, Utah in the past for a considerable period of time. In addition, precipitation and separation of the compounds particularly KCl and NaCl has created technical problems. While significant amounts of potassium sulfate are produced from these brines, the operation is not particularly efficient, since considerable processing and energy is required to recover the potassium sulfate from the other compounds.
Baniel et al. discloses in U.S. Pat. No. 2,902,341 a process for the preparation of water soluble metal sulfates, phosphates, or nitrates by the reaction in aqueous medium of the chlorides of the respective metals with free sulfuric phosphoric or nitric acid, respectively. Hydrochloric acid is extracted from the aqueous liquid with a solvent of limited mutual miscibility with water but being a solvent for hydrochloric acid but not for any of the metal salts. While this process has been exploited commercially, it lacks the simplicity and efficiency of the instant method. Large volumes of liquids must be handled; crystallization, extraction, separation, and distillation processes are required to recover the desired salts and solvents. Volatile organic solvents are utilized in the extraction process requiring stringent environmental and safety standards. The major plant utilizing this process has suffered serious fires, disrupting production for significant periods of time.
Numerous method have been taught involving recovery of potassium sulfate and other sulfates from solutions of mixed salts involving the addition of various metal alkali or sulfate salts or solution of these salts to modify the concentration and precipitate the desired products from these solutions. These processes are complex, requiring precise control of temperatures and concentrations and involve handling and recirculating large quantities of liquids. Sokolov et al., U.S. Pat. No. 4,215,100; Lampert et al. in U.S. Pat. No. 5,529,764; Efraim et al. in U.S. Pat. No. 5,552,126; Zisner in U.S. Pat. No. 5,549,876 and Neitzel et al. in U.S. Pat. No. 4,129,642; all teach processes of this nature.
Others teach methods of producing potassium sulfate by reacting sulfuric acid with potassium chloride to produce potassium acid sulfate and/or potassium sulfate. Worthington et. al in U.S. Pat. No. 4,588,573; Iwashita et. al in U.S. Pat. No. 4,342,737; Sardisco et al. in U.S. Pat. No. 4,045,543 and Myazaki in U.S. Pat. No. 4,436,710 all teach processes of this nature which require precipitation and recovery of the desired solids from the mother liquid and recirculation or evaporation of the remaining filtrate. These processes all require high energy input, many unit processes and circulation of large quantities of liquids.
Still other methods, Vajna et al., U.S. Pat. No. 4,707,347 teach recovery of potassium sulfate through ion-exchange wherein a saturated potassium chloride solution is intensively contacted with a sulfate laden anion resin to which the chloride ion is attached and the sulfate released to produce a solution of potassium sulfate from which crystals of potassium sulfate are recovered, the depleted anion exchanger being regenerated by intensive contact with magnesium sulfate solution producing a solution of magnesium chloride.
Sardisco, et al. U.S. Pat. No. 4,268,492 outline a process for the production of alkali metal sulfates from the reaction of alkali metal chlorides and sulfuric acid by way of the reaction of sulfuric acid with alkali metal fluosilicate to produce the alkali metal sulfates and fluosilicic acid H2SIF6 which reacts with the alkali metal chloride to produce hydrochloric acid and alkali metal fluosilicate.
All of the processes outlined in the prior discussion lack the simplicity, energy, and operating efficiency of the instant invention from which the finished product retains essentially the same size distribution and purity of the original solid feeds. The reaction proceeds rapidly and to completion. No external heat source is required and the process is continuous with halides being fed in the top bed and sulfates extracted from the bottom; true fluidization and gravity providing the means by which the product flows from bed to bed while counter current flow of gas (the fluidizing medium) permits the strongest sulfur dioxide gas stream to contact the most nearly converted feed while the most dilute gas contacts the raw feed thus eliminating contamination of the effluent hydrochloric acid gaseous product by the SO2 or SO3 gas.
The primary object of this invention is to eliminate the problems and inefficiencies of the prior art by providing a new process by which metal halides may be reacted with acid gases to produce dry solid compounds containing the acid gas cation while liberating a gaseous acid (or oxide) containing the metal cation.
A further object of this invention is to provide a new highly energy efficient process for said reactions.
A further object of this invention is to provide a new simple and cost effective process for the manufacture of said compounds.
A further object of this invention is to provide the conditions whereby the new method may be effectively performed to produce the said reactions.
A further object of this invention is to provide a granular product consisting of approximately the same mesh size as the initial solids which may be readily blended with existing products or which may be further granulated to permit coating with a slow release coating.
A further object of the invention is to provide a cost effective method to produce a high quality potassium sulfate fertilizer.
The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.